Stars & Their Characteristics

ConstellationsA group of stars that appear to form a

pattern in the sky.Only appear together from Earth, actually at

dif. Distances.Far distance of stars make any movement not

apparent in a constellation for thousands of years.

Asterism- small grouping of stars. Ex. Big Dipper part of constellation Ursa Major (Big Bear). Can find Polaris (North Star) using Big Dipper.

Star Movement/Sky AppearanceMovement of stars

in sky caused by Earth’s rotation & revolution.

Earth turns West to East so sky appears to move from East to West to us on Earth, that’s why the sun, moon, & stars rise in the East & set in the West.

Sky directly above poles seems stationary as Earth turns on it’s axis, so Polaris appears fixed in the sky while all other stars seem to move counter-clockwise around it. These stars are Circumpolar Stars.

Earth’s Movement & ConstellationsConstellation positions in the sky change with

the seasons. Ex. Big Dipper: Fall-near North horizon, Spring- overhead.

Some constellations can only be seen during certain seasons. Ex. Lyra in Summer & Orion in Winter.

These changes are due to Earth’s position as it orbits the Sun.

Apparent MagnitudeA measure of how bright a star appears to be

to an observer on Earth.

Magnitude is measured on a scale from 1 (brightest)- 6 (faintest).

The lower a stars magnitude #, the brighter the star.

Distances to stars3 units of measurements

1. AU (Astronomical Unit)Distance from the Sun to the Earth.

Equal to 1.5 x 108 Km or 150 million Km.

2. Light-YearMeasures the distance that a ray of light

travels in one year.

In one year, light travels 9.5 x 1012 Km or 9.5 trillion Km.

Ex: Proxima Centauri is 4.2 light years from Earth.

3. Parsec (Parallax Second)Measures distances between celestial objects

(nearest stars).One parsec = 3.258 light-years or 3.086 x

1013 Km.

Parallax- change in objects direction due to change in observers position. Occurs because Earth orbits the sun.

Using parsec measurement accounts for the shift by knowing the angel between the 2 points.

Elements in Stars• Each star is different, but they are mostly composed of H, He & some heavier elements.

•A stars spectrum is used to determine it’s composition.

•Each star has a unique spectrum.

Mass, Size & Temp. of StarsVary more in size than in mass.Mass= total amount of material in a body.

Determined by gravitational influence on other bodies.

Stellar (star) masses are expressed as multiples of the sun or One Solar Mass.

Temperature & Color of StarsRange of color a star emits depends on it’s

surface temp.To us on earth, stars look mostly white with a

bit of color.Cooler stars= reddish, Hotter stars= Bluish-

white.Ex: Iron when heated, 1st turns red, changes

to orange, to yellow to white. Very Hot objects glow Bluish.

The color/temp. of a star helps astronomers determine the elements present.

Luminosity & Absolute MagnitudeLuminosity Absolute MagnitudeActual brightness of a

star.Depends on size & temp.Ex:If 2 stars had the same

surface temp, but 1 was bigger, the bigger is more luminous.

If 2 stars were same size, but dif. temps, the hotter is more luminous

True measure of how bright a star is.

What’s the difference between apparent & absolute magnitude?

Life Cycles of Stars

Stars are born from great clouds of gas & dust. They mature, grow old, & die. When they die, they produce new clouds of dust, from which new stars may arise, along w/planets to orbit them. The more massive a star, the shorter its life will be.

What is the H-R diagram?Hertzsprung-Russell diagram – a diagram

that plots the stars surface temperature against their absolute magnitude.

Groups of the H-R Diagram:• Groups represent stages

in the life cycles of the stars.

• 90% of stars fall in the Main Sequence band & range in size & temp. these are the stars that are fusing H to He.

• Giant Stars are above main sequence & are more luminous & larger.

• Supergiants are even more luminous & large.

• White Dwarfs are near the end of their lives, were once red giants but lost atmosphere & now only glowing stellar core.

•Where in the graph are the bright, cool stars?

•Which axis represents temperature?

•What area would blue stars be located?

*The more massive a star, the shorter its life will be.

Birth of a StarA star begins its life in a cloud of gas & dust

called a Nebula.A nebula condenses when an outside force

(shockwave) acts upon it.Parts of the nebula then condense due to

gravity & the temp. increases.Large enough nebulas will begin to glow

(Protostars)Protostars get hotter & brighter eventually

fusion occurs & a star is born.

How would the amounts of H & He in the cores of 2 identical stars compare if one star was older than the other?

Death of a star like the Sun(main sequence star)Balance of fusion & gravity occurs until H is used

up & the core shrinks & releases new heat that expands the star into a Giant.

The star begins to die when temps rise & He (helium) fuses to heavier elements (C-O core)

Gas layers are blown away leaving C-O core (white dwarf) w/halo glow= Planetary Nebula.

Eventually gases dissipate into space & white dwarf is left.

Death of a Massive StarSame start as before, but fusion occurs until

Fe (iron) nuclei form & star swells more than 100 times the diameter of our sun, becoming a Supergiant.

Fe cores absorb energy & suddenly collapse sending shockwaves & outer layers out in a huge burst of light= Supernova and produces new elements.

Left behind is either a Black Hole (if original star was 15 x or more massive than our sun) or a Neutron Star.

Remnants of Massive StarsAfter a massive star “goes supernova,” it

leaves behind its core.Neutron Star: super dense, all e- are pushed

into the nuclei they orbit, result is a dense mass of neutrons that are trillions times more dense than the sun. At first, it spins & emits pulsing beams of radio waves out (Pulsar).

If star was at least 15 x more massive than the sun:

Black Hole: Intense gravitational field that not even light can escape. Detected by strong sources of x-ray (given off by atoms being ripped apart by gravity)

Life Stages of Stars: From Birth to Death

Self-Quiz1. What determines which life-cycle course a

star takes? 2. How does cloud-like nebula form

protostars?3. What process marks the transition from

protostar to star?4. What remains balanced during the long

period when a star’s H is fusing into He?5. At what point is this balance lost?

Galaxies

What Are Galaxies?Systems that contain millions or billions of

stars.

Look like hazy patches of light in the sky.

Estimated to be 50-100 billion galaxies in observable universe.

Most are millions of light years apart.

To what galaxy do we belong?Milky Way Galaxy

It’s a spiral galaxy (pinwheel-shaped)

Diameter of about 100,000 light-years

Our sun is about 26,000 light-years from it’s center

Types of galaxiesSpiral: pinwheel-shaped; nuclei of stars

bulging at center & wound spiral arms.

Elliptical: nearly spherical to oval-shaped; stars concentrated at their centers, little interstellar gas & dust for new stars to form so few to no young stars.

Irregular: irregular in shape; much smaller & fainter than other 2 galaxy types, stars spread unevenly.

Spiral Galaxy

Elliptical Galaxy

Irregular Galaxy

Spiral, Elliptical, & Irregular

Active GalaxiesEmit more energy than their stars can give

off.Some emit radiation, or change largely in

brightness in short periods of time.Possible powered by a supermassive black

hole centers that jet out hot gas in opposite directions.

Quasars: very distant, extremely luminous celestial object believed to be a type nuclei in an active galaxy.